High efficiency voltage mode class D topology

ABSTRACT

A high efficiency voltage mode class D amplifier and energy transfer system is provided. The amplifier and system includes a pair of transistors connected in series between a voltage source and a ground connection. Further, a ramp current tank circuit is coupled in parallel with one of the pair of transistors and a resonant tuned load circuit is coupled to the ramp current tank circuit. The ramp current tank circuit can include an inductor that absorbs an output capacitance C OSS  of the pair of transistors and a capacitor the provides DC blocking.

PRIORITY

This application claims the benefit of U.S. Provisional Application No.61/876,056, filed on Sep. 10, 2013 and U.S. Provisional Application No.61/968,730, filed on Mar. 21, 2014, the contents of each of which arehereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to voltage mode class Dtopologies and, more particularly, to high efficiency voltage mode classD amplifiers and wireless energy transfer systems.

2. Description of the Related Art

Recently, there have been many developments in wireless powertransmission systems (also referred to as an “energy transfer systems”)using highly resonant electromagnetic induction. In general, suchsystems include a power source and transmitting coil as well as areceiving coil connected to the device to be power (i.e., the load). Thearchitecture for wireless power transmission systems is centered on theuse of coils to generate a high frequency alternating magnetic fieldthat is used to transfer energy from the source to the device. The powersource will deliver energy in the form of voltage and current to thetransmitting coil that will create a magnetic field around the coil thatchanges as the applied voltage and current changes. Electromagneticwaves will travel from the coil, through free space to a receiving coilcoupled to the load. As the electromagnetic waves pass by and sweep thereceiving coil, a current is induced in the receiving coil that isproportional to the energy that the antenna captures.

When the source and load are coupled during wireless power transmission,the resulting configuration effectively forms a transformer with a lowcoupling coefficient. This resulting transformer has a leakageinductance that can be significantly larger than the magnetizinginductance. An analysis of the transformer model under these conditionsreveals that the primary side leakage inductance almost solelydetermines the efficiency of energy transfer. To overcome the leakageinductance, some systems use resonance to increase the voltage acrossthe leakage inductance and hence the magnetizing inductance withresulting increase in power delivery.

One conventional wireless energy transfer topology uses a traditionalvoltage mode class D (“VMCD”) amplifier in wireless energy transfersystem. FIG. 1 illustrates a circuit diagram for a VMCD amplifier. Asshown, the VMCD amplifier 100 includes a power amplifier 110 and load120. The power amplifier 110 includes two transistors 111 and 112 thatare coupled in series between a voltage source V_(DD) and ground. Thetwo transistors 111 and 112 are driven 180° out of phase to form a halfbridge topology. Conventionally, the transistors 111 and 112 can beenhancement mode, n-channel MOSFETs, for example. Furthermore, poweramplifier 110 includes a first capacitor 113 and inductor 114 that arecoupled in series with load 120 to form a resonant tuning circuit. Inthis conventional design, the power amplifier 110 tunes the load to havea resonance at the same frequency as operation of the amplifier 110.Despite zero current switching (“ZCS”), the power amplifier 110 stillexperiences high losses due to the output capacitance C_(OSS) of thetransistors 111 and 112 each time a voltage transition occurs. As thefrequency increases, the losses also increase proportionally.

To overcome these problems, existing circuits have added a matchingnetwork to the load 120 to make the load 120 appear inductive to thepower amplifier 110. For example, FIG. 2 illustrates a modified circuitof VMCD amplifier 100 illustrated in FIG. 1, but includes a matchingnetwork. As shown, VMCD system 200 includes transistors 211 and 212 andfurther includes inductor 213 and a first capacitor 214 coupled inparallel with transistor 212. Furthermore, a second capacitor 215 isconnected in series with load 220 to form a load resonant circuit 210.Due to high switching frequency and device output capacitance C_(OSS),the load resonant circuit configuration (i.e., load 220 and secondcapacitor 215) must be tuned to be inductive at operating frequency,and, therefore, allow zero voltage switching (ZVS) and correspondingreduction in output capacitance C_(OSS) losses. In design, this tuningcan lead to operation of the power amplifier 110 above resonance withdecrease in coil transmission efficiency. Although the amplifier willoperate with reduced losses (i.e., require less cooling), the improvedamplifier efficiency does not offset the reduced coil transmissionefficiency.

The matching circuit (inductor 213 and capacitor 214) functions toincrease the voltage to the load resonant circuit (capacitor 215 andload 220), which can be advantageous when limits are placed on the inputvoltage magnitude, given that the average voltage at the output of theamplifier (switch-node) is half the supply voltage V_(DD). However, thematching inductor will carry the full current of the load and thus willhave significant losses. Furthermore, the circuit is sensitive to loadresistance variation as the matching network becomes an integral part ofthe tuned resonant circuit, which can shift the ideal operatinginductance point to maintain proper ZVS.

Accordingly, a high efficiency VMCD amplifier and energy transfer systemis desired that is preferably low profile for both the source and deviceunits, easy to use, highly robust to changes in operating conditions,and does not require forced air cooling or a heat sink.

SUMMARY OF THE INVENTION

The present invention provides for a high efficiency VMCD poweramplifier that includes a pair of transistors connected in seriesbetween a voltage source and a ground connection. Further, a rampcurrent tank circuit is provided in parallel with one of the pair oftransistors. The tank circuit can include an inductor and capacitorconnected in series and is provided to collectively absorb an outputcapacitance C_(OSS) of each of the pair of transistors. Preferably, theL-C network of the tank circuit is designed with a very low resonantfrequency, such that the converter operates as a no load buck converter.The L-C network only encounters ripple current, but does not incurlosses relating to the load. As a result, inductor sizes can remainsmall and losses minimized. In one to refinement of the invention, thehigh efficiency VMCD power amplifier includes a plurality of rampcurrent tank circuits coupled in parallel that enable discreteprogrammability of the ZVS current (i.e., a “ZVS VMCD power amplifier”).The VMCD power amplifier can be implemented in a wireless energytransfer system.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present disclosure willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly elements and in which:

FIG. 1 illustrates a circuit diagram for a conventional voltage modeclass Damplifier.

FIG. 2 illustrates a conventional VMCD amplifier implemented with amatching network.

FIG. 3 illustrates a high efficiency VMCD amplifier according to anembodiment of the present invention.

FIG. 4 illustrates a high efficiency wireless power VMCD systemaccording to an embodiment of the present invention.

FIG. 5A illustrates a theoretical waveform for the switching devices ofthe energy transfer system illustrated in FIG. 4.

FIG. 5B illustrates a theoretical waveform of the tank circuitcomponents of the energy transfer system illustrated in FIG. 4.

FIG. 6 illustrates a measured system efficiency of the energy transfersystem illustrated in FIG. 4 with eGaN FETs.

FIG. 7 illustrates a simulation of a Figure of Merit comparison betweenexemplary embodiment of the energy transfer system illustrated in FIG.4.

FIG. 8 illustrates a simulated comparison between total FET power forthe VMCD comparison between the GaN transistors and MOSFETs.

FIGS. 9A-C illustrate alternative embodiment of a high efficiency VMCDamplifier according to an exemplary embodiment of the present invention.

FIG. 10 illustrates a VMCD amplifier in accordance with anotherembodiment of the present invention.

FIG. 11 illustrates another exemplary embodiment of a VMCD amplifier inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description, reference is made to certainembodiments. These embodiments are described with sufficient detail toenable those skilled in the art to practice them. It is to be understoodthat other embodiments may be employed and that various structural,logical, and electrical changes may be made. Moreover, while specificembodiments are described in connection with energy transfer systems, itshould be understood that features described herein are generallyapplicable to other types of circuits, such as RF amplifiers and thelike.

FIG. 3 illustrates a high efficiency VMCD amplifier according to a firstembodiment of the present invention. As shown, VMCD amplifier 300includes two transistors 311 and 312 coupled in series between a voltagesource V_(DD) and ground, forming a half bridge topology. In theexemplary embodiment, transistors 311 and 312 are enhancement mode,n-channel MOSFETs. However, it should be understood that the inventionis not so limited. As will be described in more detail below, VMCDamplifier 300 preferably uses GaN FETs in an alternative embodiment.Although not shown, it should be appreciated that a control circuit,such as an oscillator, is coupled to the gates of transistors 311 and312 to alternatively turn on the first transistor 311 and the secondtransistor 312.

As further shown, VMCD amplifier 300 includes a resonant tuning circuit320 formed from capacitor 321 and inductor 322, which are coupled inseries between the switch node (i.e., the node between the source oftransistor 311 and drain of transistor 312) and the load 340. VMCDamplifier 300 also includes a ramp current tank circuit 330 coupledbetween the switch node and ground, i.e., coupled in parallel totransistor 312. The ramp current tank circuit includes inductor 331 andcapacitor 332, which are provided to collectively absorb the outputcapacitances C_(OSS) of the transistors 311 and 312 by providing acurrent that will allow the circuit to self-commutate the switch-nodewith the necessary dead-time between the gate signals applied totransistors 311 and 312. Preferably, the L-C network of the tank circuitis designed with a very low resonant frequency, enabling the converterto effectively operate as a no load buck converter. The L-C network onlyencounters ripple current, but does not incur losses relating to theload, as experienced by conventional systems. As a result, inductorsizes can remain small and losses minimized. By keeping losses to aminimum, the L-C network functions to ensure zero voltage switching(“ZVS”) operation of the amplifier, which can advantageously be used tosupport the operation of capacitively tuned load coils, and, moreimportantly, coils with wide load ranges that can alternate between aninductive load and a capacitive load. Furthermore, in the exemplaryembodiment, VMCD amplifier 300 is preferably designed with the loadresonance tuned to the operating frequency to further improve efficiencyof energy transfer.

FIG. 4 illustrates an exemplary embodiment of a high efficiency wirelesspower VMCD system that includes a power transmitting device and a powerreceiving device. As shown, FIG. 4 illustrates an energy transfer systemin which the power transmitting device includes VMCD amplifier 300illustrated in FIG. 3. Namely, the power transmitting device includestransistors 411 and 412 coupled in series between a voltage sourceV_(DD) and ground. Further, the power transmitting device comprises aramp current tank circuit 430, which includes inductor 431 and capacitor432, coupled between the switch node and ground. Capacitor 413 isconnected in series with coil 414 to collectively form the powertransmitting device 401. Although not shown, it should be appreciatedthat a control circuit, such as a dead-time control module, is coupledto the gates of transistors 411 and 412 to alternatively turn on thefirst transistor 411 and the second transistor 412.

When the power receiving device, including load 440, is inductivelycoupled to the power transmitting device, a highly resonant wirelessenergy transfer coil with matching network 460 is formed between the twodevices. The power receiving device includes diodes 451, 452, 453 and454 and a capacitor 455 coupled between diodes 452 and 453, whichcollectively function as a rectifier as would be understood to oneskilled in the art. Furthermore, the power receiving device includescapacitors 425, 426 and coil 427 that collectively form the matching andresonant tuning network with capacitor 413 and coil 414 of the powertransmitting device. Preferably, the inductances of inductor 431 isselected with a small value designed of offset the device capacitanceC_(OSS). Furthermore, the capacitor value of capacitor 432 can beselected for dynamic load requirements if needed.

FIG. 5A illustrates a theoretical waveform for each of the switchingdevices (i.e., transistors 411 and 412) of the energy transfer system400 illustrated in FIG. 4. As shown, the drain source voltage V_(DS) isdirectly related to the square wave signal of the voltage source V_(DD)and the drain current I_(D) of the device is a function of drain sourcevoltage V_(DS). FIG. 5B illustrates a theoretical waveform of thecomponents inductor 431 and capacitor 432 of the tank circuit 430.Again, the drain source voltage V_(DS) is shown to be equal to thesquare wave signal of the voltage source V_(DD). Further, currentI_(LZVS) through inductor 431 and current I_(Load) through load 440 aredependent on drain source voltage V_(DS) of the device. As should beappreciated, load variations will have only a minimal impact on the tankcircuit as long as the deviations in the load current remain below thepeak current in inductor 431. Accordingly, the energy transfer system400 ensures proper switching for the devices and maintains low lossesfor the devices. As such, the only other factor that can influence theoperation of the energy transfer system 400 is the supply voltage V_(DD)since the current in inductor 431 is supply dependent.

As described above and shown in FIGS. 3 and 4, VMCD amplifier 300 andenergy transfer system 400 comprises transistors 311, 312 and 411, 412,respectively, which are enhancement mode, n-channel MOSFETs in theexemplary embodiment. Preferably, transistors 311, 312 and/or 411, 412are GaN FETs, such as EPC2007 devices manufactured by Efficient PowerConversion Corporation.

An experimental comparison between two exemplary energy transmissionsystems, one with enhancement mode, n-channel MOSFETs and one with GaNFETs, reveals that GaN transistors will have a larger impact onconverter efficiency at lower output power levels and will be describedas follows.

First, referring back to FIG. 4 for example, the wireless energytransfer system 400 can be provided with GaN transistors as transistors411 and 412. Moreover, inductor 431 can be provided with a value of 300nH and capacitor 432 can be provided with a value of 1 μF withcorresponding dead-time (V_(TH) to V_(TH)) of 3.2 ns at 36 V input.Moreover, in this exemplary embodiment, the coil set can be tuned toresonance with C_(s) at the operating frequency. Experimental andanalytical results of this configuration will be described as follows.

FIG. 6 illustrates the measured system efficiency (input supply tooutput load) for the comparative example, including gate power for a35.4 Ω load and 23.6 Ω load. As shown, the system efficiency peaks at83.7% with 36.1 W load power for the 23.6 Ω case.

FIG. 7 illustrates a simulation of a Figure of Merit (“FOM”) comparisonbetween the exemplary embodiment having GaN transistors and an exemplaryenergy transfer system using MOSFETs. In this experimental comparativeanalysis, FDMC8622 n-channel MOSFETs manufactured by FairchildSemiconductor® are selected since these devices have a similar Q_(OSS)value and the same voltage rating as the EPC2007 GaN transistors.

Generally speaking, it should be appreciated that ZVS voltage mode classD topology is considered a class of soft switching converter.Accordingly, FIG. 7 illustrates a comparison of the soft switching FOMbetween the devices each used in the configuration of the exemplaryenergy transfer system 400 illustrated in FIG. 4. As shown in FIG. 7,there is no appreciable difference in system efficiency between the GaNFET design and the MOSFET design. This results from the way in which thecapacitor output C_(OSS) is absorbed and the tradeoff between R_(DS(on))and the timing and magnitude of the tank circuit impact on devicelosses.

However, FIG. 8 illustrates a comparison between total FET power(includes gate power) for the VMCD comparison between the GaNtransistors and MOSFETs. As shown, the difference between the GaNtransistors and MOSFETs is based on gate power consumption and revealsthat GaN transistors have a larger impact on converter efficiency atlower output power levels. The total device power difference is nearconstant at around 900 mW over the entire load power range.

FIGS. 9A-C illustrate alternative embodiments of a high efficiency VMCDamplifier according to the present invention. In particular, the VMCDamplifiers illustrated in FIGS. 9A-C comprise similar components as VMCDamplifier 300 illustrated in FIG. 3, except in these embodiments, theVMCD amplifier includes additional tank circuits, enabling discreteprogrammability of the ZVS current (i.e., “ZVS VMCD power amplifiers”).

As shown in FIG. 9A, two transistors 911 and 912, preferably GaN FETs,are provided that are coupled in series between a voltage source V_(DD)and ground. Further, the VMCD amplifier includes a resonant tuningcircuit 920 formed from capacitor 921 and inductor 922, which arecoupled in series between the switch node and the load 940. Theexemplary VMCD amplifier also includes a primary tank circuit 930coupled between the switch node and ground, i.e., coupled in parallel totransistor 912. The ramp current tank circuit includes inductor 931 andcapacitor 932. It should be appreciated that these components have thesame configuration as VMCD amplifier 300 illustrated in FIG. 3. Althoughnot shown, it should be appreciated that a control circuit, such as adead-time control module, is coupled to the gates of transistors 911 and912 to alternatively turn on the first transistor 911 and the secondtransistor 912.

Furthermore, the VMCD amplifier illustrated in FIG. 9A includes one ormore secondary tanks circuits 950 . . . n coupled in parallel to theprimary tank circuit 930. As shown, a first add-on ZVS tank circuit 950that includes inductor 951 and capacitor 952 is connected in parallel tothe primary tank circuit. An additional transistor 953 is connected inseries between capacitor 952 and ground. It is contemplated that theVMCD amplifier can include n add-on ZVS tank circuits, with the nth tankcircuit in FIG. 9A illustrated to include inductor L_(n) and capacitorC_(n) and transistor Q_(n+2). It should be appreciated that thisconfiguration of the primary tank circuit 930 in parallel with n add-onZVS tank circuits collectively absorb the output capacitances C_(OSS) ofeach transistor in the circuit, including transistors 911, 912, 953,Q_(n+2) and so forth.

FIG. 9B illustrates a modification of the high efficiency VMCD amplifiershown in FIG. 9A. As shown in FIG. 9B, the high efficiency VMCDamplifier includes many of the same components as the design of FIG. 9A,including transistors 911 and 912, resonant tuning circuit 920, andprimary tank circuit 930 coupled between the switch node and ground andincluding inductor 931 and capacitor 932. As further shown, eachsecondary tank circuit is coupled in parallel to inductor 931 of primarytank circuit 930. Moreover, capacitor 952 (shown in FIG. 9A) is replacedwith transistor 953. Again, it is contemplated that the VMCD amplifierof FIG. 9B can include n add-on ZVS tank circuits, with the nth tankcircuit in FIG. 9B illustrated to include inductor L_(n) and transistorQ_(n+2). Each of the n add-on ZVS tank circuits is coupled in parallelto inductor 931 of the primary tank circuit 930.

FIG. 9C illustrates a modification of the high efficiency VMCD amplifiershown in FIG. 9B. In this embodiment, the components are the same asthat in FIG. 9B except that the connection of inductor 931 and capacitor932 of the primary tank circuit 930 is reversed. In other words,capacitor 932 is coupled to the switch node between transistors 911 and912 and inductor 931 is coupled in series between capacitor 932 andground. Similar to the embodiment illustrated in FIG. 9B, each secondarytank circuit is coupled in parallel to inductor 931 of primary tankcircuit 930. It is again contemplated that the VMCD amplifier of FIG. 9Ccan include n add-on ZVS tank circuits, with the nth tank circuit inFIG. 9B illustrated to include inductor L_(n) and transistor Q_(n+2).Each of the n add-on ZVS tank circuits is coupled in parallel toinductor 931 of the primary tank circuit 930.

Also, it should be understood to one skilled in the art that the VMCDamplifier illustrated in FIGS. 9A-C can be implemented in a highefficiency wireless power VMCD system with a similar designconfiguration that VMCD amplifier 300 of FIG. 3 is utilized in theenergy transfer system 400 of FIG. 4.

FIG. 10 illustrates a VMCD amplifier 1000 in accordance with anotherembodiment of the present invention. As discussed above, VMCD amplifier300 with transistors 311 and 312 form a half bridge topology. VMCDamplifier 1000 illustrated in FIG. 10 comprises four transistors,preferably GaN FETs, to form a full bridge topology. Although not shown,it should be appreciated that in one embodiment, a control circuit, suchas a dead-time control module module, can be coupled to the gates of thetransistors to alternatively turn on and turn off the transistors aswould be understood to one skilled in the art.

As shown in FIG. 10, VMCD amplifier 1000 includes transistors 1011 and1012 coupled in series between the voltage source V_(DD) and ground. Twoadditional transistors 1013 and 1014 are coupled in parallel totransistors 1011 and 1012, also between the voltage source V_(DD) andground. The four transistor design forms a full bridge configuration forVCMD amplifier 1000. As would be appreciated to one skilled in the art,the full bridge configuration doubles the output power for the voltagesource V_(DD) applied to a half bridge configuration, such as the designillustrated in FIG. 3. Preferably, transistors 1011 and 1014 switchtogether and transistors 1012 and 1013 switch together during operation.

Furthermore, the full bridge configuration eliminates a capacitor fromthe tank circuit provided in the half bridge topology of VCMD amplifier300 of FIG. 3. As shown in FIG. 10, inductor 1015 is coupled betweenswitch nodes N1 and N2 in parallel to load 1020. Capacitor 1016 isprovided for resonant toning with load 1020. It should also beappreciated that the design shown in FIG. 10 has the load 1020 connecteddifferentially, which also advantageously reduces possibleelectromagnetic interference.

Finally, FIG. 11 illustrates yet another exemplary embodiment of a VMCDamplifier in accordance with the present invention. As shown in FIG. 11,the high efficiency VMCD amplifier includes many of the same componentsas the designs of FIGS. 9A-9C, including transistors 911 and 912 andresonant tuning circuit 920. In this embodiment, the primary tankcircuit 1130 includes inductor 1131 and a pair of transistors 1113 and1114. Collectively, inductor 1131 and transistor 1114 are coupled inseries and further coupled between the switch node and ground.Furthermore, transistor 1113 is coupled between the voltage sourceV_(DD) and a node connecting inductor 1131 and transistor 1114. In thisconfiguration, it should be appreciated that the resonant tuned load 920is coupled to a half bridge topology that enables two distinct loads tobe used, such that tuning between the coils can be slightly shiftedfacilitating significant power bandwidth due to the load and couplingvariation.

The above description and drawings are only to be consideredillustrative of specific embodiments, which achieve the features andadvantages described herein. Modifications and substitutions to specificprocess conditions can be made. Accordingly, the embodiments of theinvention are not considered as being limited by the foregoingdescription and drawings.

What is claimed is:
 1. A power amplifier comprising: a pair oftransistors connected in series between a voltage source and a groundconnection; a switch node disposed between a source of a firsttransistor of the pair of transistors and a drain of a second transistorof the pair of transistors; a non-resonant tank circuit connectedbetween the switch node and the ground connection or supply connectionor both, the tank circuit having an inductor with an inductance thatabsorbs an output capacitance of the pair of transistors, and acapacitor connected in series with the inductor and ground, thecapacitor having a capacitance to provide DC blocking, wherein the tankcircuit enables the power amplifier to operate as a no load buckconverter with zero voltage switching; and a resonant tuning circuitconnected in series between the switch node and a load coupled to thepower amplifier; wherein the power amplifier is configured toself-commutate the switch node with a necessary dead-time between gatesignals applied to the pair of transistors.
 2. The power amplifier ofclaim 1, wherein each of the pair of transistors is a GaN transistor. 3.The power amplifier of claim 1, further comprising at least one add-ontank circuit coupled in parallel with the tank circuit.
 4. The poweramplifier of claim 3, wherein the at least one add-on tank circuitcomprises a second inductor and a second capacitor.
 5. The poweramplifier of claim 4, further comprising another transistor connected inseries between the at least one add-on tank circuit and the groundconnection.
 6. The power amplifier of claim 3, wherein the at least oneadd-on tank circuit comprises: a second inductor having a pair ofterminals with a first terminal coupled to the switch node; and anothertransistor having a source coupled to a node between the inductor andthe capacitor of the tank circuit and a drain coupled to a secondterminal of the second inductor.
 7. The power amplifier of claim 3,wherein the at least one add-on tank circuit comprises: a secondinductor having a pair of terminals with a first terminal coupled to anode between the inductor and the capacitor of the tank circuit; andanother transistor having a source coupled to the ground connection anda drain coupled to a second terminal of the second inductor.
 8. Awireless energy transfer system comprising: a power transmitting deviceincluding: a pair of transistors connected in series between a voltagesource and a ground connection; a switch node disposed between a sourceof a first of the pair of transistors and a drain of a second of thepair of transistors; a non-resonant tank circuit connected between theswitch node and the ground connection, the tank circuit having aninductor with an inductance that absorbs an output capacitance of thepair of transistors, and a capacitor connected in series with theinductor and the ground connection, the capacitor having a capacitanceto provide DC blocking, wherein the tank circuit enables the poweramplifier to operate as a no load buck converter with zero-voltageswitching, and wherein the power transmitting device is configured toself-commutate the switch-node with a necessary dead-time between gatesignals applied to the pair of transistors; and a power receiving deviceincluding: a load; a rectifier connected in parallel to the load; a pairof capacitors coupled to the rectifier; and a receiving coil coupled inparallel to at least one of the pair of capacitors.
 9. The wirelessenergy transfer system of claim 8, wherein when the power receivingdevice is inductively coupled to the power transmitting device a highlyresonant wireless energy transfer coil with matching network is formed.10. The wireless energy transfer system of claim 8, wherein each of thepair of transistors is a GaN transistor.
 11. The wireless energytransfer system of claim 8, further comprising at least one add-on tankcircuit coupled in parallel with the tank circuit.
 12. The wirelessenergy transfer system of claim 11, wherein the at least one add-on tankcircuit comprises a second inductor and a second capacitor.
 13. Thewireless energy transfer system of claim 12, further comprising anothertransistor connected in series between the at least one add-on tankcircuit and the ground connection.
 14. The wireless energy transfersystem of claim 11, wherein the at least one add-on tank circuitcomprises: a second inductor having a pair of terminals with a firstterminal coupled to the switch node; and another transistor having asource coupled to a node between the inductor and the capacitor of thetank circuit and a drain coupled to a second terminal of the secondinductor.
 15. The wireless energy transfer system of claim 11, whereinthe at least one add-on tank circuit comprises: a second inductor havinga pair of terminals with a first terminal coupled to a node between theinductor and the capacitor of the tank circuit, and another transistorhaving a source coupled to the ground connection and a drain coupled toa second terminal of the second inductor.